1. Field of the Invention
The present invention relates to NMR (nuclear magnetic resonance) equipment and, more particularly, to a wire member for use in a detection coil incorporated in an NMR probe that directs RF waves at a sample and detects RF waves arising from the sample. The invention also relates to a method of fabricating this wire member.
2. Description of Related Art
In the field of chemical analysis, high-resolution NMR spectroscopy has been widely used. NMR provides a means that is quite useful in determining the molecular structures of samples.
Normally, proton 1H and carbon 13C are used to determine the molecular structures by NMR. These isotopes have chemical shifts reflecting the surrounding atomic arrangement. The chemical shifts can be observed as NMR frequency differences. Determination of a molecular structure is performed by investigating the spatial coupling between atoms based on the observed chemical shift.
FIG. 4 is a block diagram schematically showing NMR equipment used for NMR spectroscopy. The NMR equipment, generally indicated by reference numeral 20, comprises a main magnet 21 for producing a static magnetic field to be applied to a sample 27, an NMR spectrometer 22 for performing NMR spectroscopy, a power amplifier 23 for power-amplifying RF pulses delivered from the NMR spectrometer 22, a preamplifier 24 for preamplifying a detection signal applied to the NMR spectrometer 22, and a duplexer 25 for duplexing and/or unduplexing the power amplifier 23 or preamplifier 24 signals.
FIG. 5 shows a probe for use in the NMR equipment described above. Normally, a sample 31 is dissolved in deuterium or a deuterated organic solvent and held in a glass test tube 32. The main magnet 21 (FIG. 4) applies a uniform static magnetic field to the sample 31. The probe 30 is fitted with a detection coil 33 and mounted close to the test tube 32.
An RF pulse having a frequency almost equal to the resonance frequency is applied to the sample 31 from the NMR spectrometer 22 (FIG. 4) via the power amplifier 23. The sample 31 produces an RF magnetic field, or an NMR signal, in resonance with the magnetic-field component of the RF pulse. The NMR signal is detected as an induction current through the detection coil 33. The detected current is sent to the NMR spectrometer 22 via the preamplifier 24.
Signals from atomic nuclei contained in the sample 31 can be discriminated based on the NMR frequencies received by the NMR spectrometer 22. In many cases, atomic nuclei to be investigated are protons 1H. Chemical shifts due to the atomic arrangement around the protons 1H exist.
Chemical shifts cover a range on the order of ppm. The width of an NMR signal is approximately 10−3 ppm. To separate signals well, the inhomogeneity of the static magnetic field applied to the sample 31 within the test tube 32 needs to be small. If the inhomogeneity of the static magnetic field applied to the sample 31 increases, the NMR signal will be broadened, and the instrumental resolution will deteriorate.
Therefore, the NMR equipment 20 (FIG. 4) uses the main magnet 21 to produce a uniform magnetic field accurately. Usually, a current shim set 26 capable of producing a desired magnetic field gradient is mounted between the main magnet 21 and probe 30. The current through the current shim set 26 is controlled by the NMR spectrometer 22 to cancel the residual inhomogeneous component of the main magnetic field. That is, shimming is done. The resolution adjustment using the shimming is quite complex, which makes operations on the NMR instrument quite difficult.
The probe 30 is inserted within a uniform magnetic field produced by the aforementioned technique to detect the NMR signal produced from the sample 31. However, if the probe 30 has a magnetic property, the surrounding uniform static magnetic field is perturbed.
Since the detection coil 33 of the probe 30 is placed closest to the sample 31, the static magnetic field near the sample 31 is affected conspicuously by the magnetic susceptibility of the detection coil 33. Often, magnetic field distortion by the effects of such a close magnetic field cannot be removed by the outside current shim set 26. If an inappropriate material is selected as the detection coil 33, it is quite difficult to correct the instrumental resolution. For this reason, a material (e.g., copper or aluminum) not including any ferromagnetic material, such as iron, and possessing a small magnetic susceptibility is used in the probe 30.
Preferably, the Q value of the detection coil 33 is set higher in securing a high sensitivity in detecting signals. Therefore, the detection coil 33 is made of a material having a high electrical conductivity, such as copper.
To reduce magnetic effects, the detection coil 33 is made of a thin or slender material. However, a high electrical conductivity needs to be secured in order to secure a high Q value. Therefore, the material has a sufficient thickness. Accordingly, the suppression of magnetic effects and securing a high electrical conductivity are conflicting requirements.
Materials that satisfy these requirements about the detection coil 33 (i.e., are low in magnetic susceptibility and high in electrical conductivity) have been heretofore proposed. For instance, they are combinations of two or more kinds of pure metals having mutually canceling magnetic susceptibilities. One example is copper wire having an aluminum core. Another example consists of two pieces of copper foil between which a piece of aluminum foil is sandwiched.
These materials do not result in a decrease in the electrical conductivity. However, these different metals are different in hardness or processed to different levels of hardness. It is difficult to wire draw or roll combinations of them uniformly. Cross sections of these raw materials tend to lack uniformity. Therefore, the magnetic susceptibility is distributed nonuniformly, in the same way as in the first-mentioned case. This impairs the magnetic field uniformity.
Furthermore, it has been experimentally found that it is quite difficult to avoid adhesion of the metallic iron component introduced during a wire-drawing process or rolling process, the component being created by a machine tool (C. M. Hurd, Cryogenics, October 1966, pp. 264-269). The adhesion of the metallic iron component to raw materials can be checked by electron microscopy.
Adhesion of metallic iron to a raw material is briefly described. As an example, copper is a diamagnetic material. After undergoing a wire-drawing or rolling process, the magnetic susceptibility of the inherent diamagnetism often decreases by more than 60%. Furthermore, the magnetic susceptibility varies greatly among measurement locations. The sign of the magnetic susceptibility is frequently inverted and even paramagnetism may be exhibited, because the adhering metallic iron produces a magnetic field that is 106 times stronger than the field produced by the same weight of copper.
Metallic iron severely affecting the magnetic property of the raw material in this way can be regarded as a magnetic contaminant for the raw material. However, the weight ratio to the base material is only less than 10 ppm, which is less than the amount that can be identified as an impurity by chemical analysis. Therefore, this by no means infringes the industrial standards regarding purity. For example, where a spot of iron corresponding in weight to 10 ppm adheres to a portion 1 cm long, a local magnetic field that cannot be corrected by the current shim set 26 is produced. This fatally deteriorates the resolution of the NMR instrument.